1. Field of the Invention
The present invention relates to an image display medium, device, and method, and more particularly to an image display medium, device and method using movable particles.
2. Description of the Related Art
Conventionally, display technologies such as Twisting Ball Display (rotational display by particles colored separately into two colors), or using electrophoresis, magnetophoresis, thermal rewritable medium, liquid crystals having a memory function and the like have been proposed for a sheet-like image display medium which is repeatedly rewritable. Among the above described display technologies, although thermally rewritable mediums and liquid crystals having memory functions where excellent memory functions for images, display surface color cannot be made as that of white as white paper. Thus, it is difficult to visually discern confirm a distinctions between image parts and non-image parts when certain images are displayed, that is, there has been a problem of poor image quality. Other display technologies using electrophoresis or magnetophoresis are those which, and colored particles having are memory function for image dispersed in a white liquid. Thus, the display technologies using electrophoresis or magnetophoresis are excellent for displaying white colors. However, there has been a problem of poor image quality, since the white liquid enters between colored particles, resulting in black color image parts appearing grayish. Moreover, since white liquid is enclosed inside an image display medium, there is a possibility that the white liquid will leak from the image display medium, if the image display medium is removed from an image display unit and handled roughly, like a paper. Another technology, Twisting Ball Display has memory functions, uses oil. The oil exists substantially in a solid state only in cavities around particles inside an image display medium. Thus it is comparatively easy to construct the image display medium in the form of a sheet. However, even if each hemispherical surface of the particles has been coated in white and perfectly aligned at a side of the display, light entering between the spherical the particles is not reflected and lost inside the display. Thus, in principle, a white display having a coverage of 100% cannot be achieved, resulting in a slightly grayish appearance when the color should be white. Further, since particle size is required to be smaller than pixel size for obtaining high resolution, particles coated with different colors thereon must be made smaller. Hence an advanced manufacturing technique is required.
In order to solve the problems as described above, there has been proposed a similar display technology using toner. Colored conductive toner particles and white particles are enclosed between opposed electrode substrates. When the colored conductive toner particles are charged by contact with the electrode substrates, the charged colored toner particles move toward the substrate at the side of the display surface through non-charged white particles due to an electric field established between the electrode substrates. The colored toner particles are deposited on the inside of the substrate at the display surface side, whereby an image is displayed using difference in contrast between the colored toner particles and white particles. The present display technology is advantageous in that where a whole image display medium is composed of solid materials, the display can be switched between black and white colors with a coverage of 100% from each other, in principle. Furthermore, a technology for displaying a color image by using a laminated display medium in which colored particles forming a chain structure via a electric field are dispersed in an electrically insulating solvent has been disclosed in Japanese Patent Laid-Open No. 101409/1996. By using the technology described above, representing with three colors is possible, but leakage cannot be avoided as there is liquid enclosed in the display medium. In addition, if this technology is used, power must be constantly applied to the display medium for effecting light transmission, which results in poor memory function.
According to the display technology using toner particles, a displayed image can be stored without any power being applied. Moreover, the display medium is easily handled and manufactured, so that there are additional advantages relative to the technology using electrophoresis. However, an image is displayed with two colors by moving a single kind of charged colored particles via an electric field, and hence the image cannot, in principle be display with three or more colors in a single display cell.
Furthermore, in a display unit using toner particles, even when it is intended to use a technology displaying multiple colors in combination with laminated cells, which cells can represent colors different from each other, toner particles having different colors exist on the side of a substrate surface opposed to the display surface, therefore, it is difficult to display a color in a lower layer by transmission through an upper layer having another color. Generally, in a laminated-type display panel, since lamina of each layer is for each different color of the display, a sense of disorientation or vertigo can arise when multiple colors are displayed. Thus, as much as possible, it is preferred that such multiple colors be displayed on a single surface.
In another color display technology, cells each having a different color are arranged adjacent to each other. In this respect, however, there is an inverse relationship between the number of cells that can be combined into a single pixel, and the number of colors that can be displayed, resulting in a limited resolution. Accordingly, it is desirable to display as many colors as possible in a single cell.
The present invention has been made in view of the background described above, so that an object of the present invention is to provide an image display unit, an image display medium, and an image display controller, wherein a plurality of colors are allowed to develop selectively with groups of particles enclosed in a space defined between a pair of substrates.
In order to attain the above object, an image display medium comprises a pair of substrates, with one or more of the substrates being at least translucent or substantially transparent, and space defined between them with at least one of a gas disposed in the space, or a partial vacuum formed in the space; and a plurality of types of particle groups enclosed in the space defined between said pair of the substrates, each particle type having a charge of a polarity identical in polarity to all of the other types of particle groups, and the types of particle groups having an adhesiveness with respect to the substrates different from one another.
Preferably, at least one of the pair of the substrates is substantially transparent, with gas enclosed in the space defined between the pair of the substrates, or a partial vacuum formed in the space, or some combination thereof.
The plurality of types of particle groups are enclosed in the space defined between the pair of the substrates. The plurality types of particle groups have different colors from one another, with each type having a charge of a polarity identical to all of the other types of partical groups, and an adhesiveness with respect to the substrates which is allowed to differ from one another. An image display medium comprises the pair of the substrates and the plurality of types of particle groups.
An electric field generator is connected to the substrates for applying an electric field having an intensity according to the average charge amount of a type of particle group for transffering that type of particle group to one of the substrates for displaying a particular color.
In the meantime, the adhesiveness of the particle groups with respect to the substrates indicates the degree of difficulty in separating respective particles in the particle groups away from the substrates. In this connection, the adhesiveness of particle groups with respect to substrates is determined specifically by at least one factor of an average charge amount per particle, an average diameter of particles, an average degree of sphericity per particle, and an average degree of surface roughness per particle and taking such factors into consideration when an electric field is applied by the electric field generator. Namely, when particles adhere to substrates, van der Waals force (intermolecular force) functions between the substrates and the particles. This force corresponds to contact areas defined by particles in contact with the substrates. The larger the contact areas become, the more intensive the force becomes, and the adhesiveness (difficulty in separating particles) becomes greater. The contact area corresponds to a diameter of particle, average degree of sphericity per particle, and an average degree of surface roughness per particle. On the other hand, particles have been charged as mentioned above, and when an electric field is applied by an electric field generator to the particles, electric or Coulomb attraction acts upon the particles. The Coulomb attraction varies with an amount of charge on the particles.
It is to be noted that the average diameter of particles, average charge amount per particle, average degree of sphericity per particle, and average degree of surface roughness per particle are defined by, for example, the following measurements;
Average charge amount: Blowing off or adherence of an electric field to an insulation film
Average diameter of particle: A value of 50% in volume measured by a particle counter
Average degree of sphericity: Out-of-roundness in a projected image of a particle measured by a microscope
Average surface roughness: (a peripheral length of a projected image of a particle measured by a microscope)/(a peripheral length of a circle having the same area as that of the projected image)
Furthermore, a diameter of a particle, a degree of sphericity of a particle, and surface roughness of a particle are specified by method of pulverization or a chemical technique. Namely, particles used in the present invention are prepared by usual methods for preparing fine particles as described in xe2x80x9cFine Particle Industryxe2x80x9d edited by the Technical Association of Japan Powder Industry: Asakura Publishing Co., etc.
A first method is for mechanically preparing particles from a large bulk. The particles each has a desired particle diameter and surface condition by means of compression, mechanical shock, and shear crushing; pulverization of the particles by the use of roll mill, pin mill, and jet mill; classification of the particles by means of mechanical, and jetting method; and adjustment of a surface condition of the classified particles by means of a compounding device.
A second method is a method in which uniform, spherical individual particles or fine particle agglomerates are chemically prepared by means of emulsion polymerization and the like, and then the resulting materials are classified by sedimentation, centrifugal separation or the like.
Meanwhile, the present inventors have found that a display medium in which charged particles are used, exhibits the following characteristic features in the course of accomplishing the present invention. Namely, in the display medium in which charged particles are employed, adhesion of the particles to substrates and separation of the particles from the substrates can be rapidly effected as compared with a medium in which a solvent is used in electrophoresis. Since this type of a medium exhibits threshold characteristics with respect to an electric field strength, it can be precisely controlled by means of the intensity of electric field strength. More specifically, in the display medium using the particles, the particles adhere to and are immobile from the surface of substrates before the electric field reaches a certain level, but when it exceeds the level, the particles are separated from the substrates and can move to the opposing substrate at a high speed. This is because the particles are surrounded by a gas or in a substantial vacuum, so that it is considered that the particles which were once released from adhesion move easily to the opposed substrate. For this reason, when it is intended to release the particles, transfer of the particles can be accurately adjusted by controlling the electric field strength. Furthermore, adhesiveness of particles can be also desirably changed by selecting particle configuration or particle diameter in addition to the control of an amount of charge of the particles.
On the other hand, when an electrophoretic method is used, since an insulating solvent exists around particles, a moving speed of particles becomes extremely slow due to strong viscosity resistance, so that it is unsuitable for high-speed operation required in a display unit. Moreover, variations in adhesiveness of particles to the substrate when an electric field is not applied are large, so that no threshold value for particle movement can not be determined with respect to an electric field. Concerning memory function, when the solvent is oscillated, a large dynamic force is directly applied to particles, so that the particles are separated from or contact with each other due to uncontrollable environmental factors by a user.
Therefore, in a plurality of such particle groups enclosed in a gas or vacuum, having different colors, having the same polarity, and making adhesiveness to the substrate different from one another, the movement of the particle groups can be selectively and precisely adjusted by controlling an electric field strength, because of the reasons mentioned above. As a consequence, according to the present invention, a plurality of colors can be selectively developed to attain color display by means of particle groups enclosed in space defined between a pair of substrates.
Three or more types of particles having the same polarities can be enclosed in the space. According to convenience in developing color or other like reasons, such a particle group having substantially the same degree of electrostatic adhesiveness and a different color may further be added. In other words, a desired color is produced by the use of the particle group having substantially the same degree of electrostatic adhesiveness and a different color. In this case, when one particle group is moved, the particle group added further is simultaneously moved.
Moreover, a particle group having a different polarity of charge from that of the above described particle groups is enclosed in the space together with them, that is, at least one particle group having of a different polarity of charge from that of the plural types of particle groups enclosed in the space between the substrates. As a result, it becomes possible to display further richer colors. Particularly, when the at least one particle group has a white color (the above described plural types of particle groups do not contain white particle group in this case), it becomes possible to display an image having a high contrast as of a color image that was printed on a white paper. In this case, it may be arranged such that the at least one particle group can be a plurality of types of particle groups which have adhesiveness to substrates different from one another as in the aforementioned plural types of particle groups.
Furthermore, adhesiveness of each of the above described plural types of particle groups may be allowed to have a predetermined distribution breadth. According to such modification, it becomes possible to continuously change a density thereof. In this case, however, when the distribution breadth overlaps that of another type of particle group, it results in turbidity of color in the overlapping region. Accordingly, a portion of particles existing in such overlapped region is desirably, for example, 30% or less. The overlapped region is desirably in a cell, with overlapping as low as possible. The distribution breadth in electrostatic adhesiveness can be determined by suitably selecting a particle diameter distribution for particles to be used.
In the meantime, it may be arranged in such that a plurality of a pair of substrates enclosing the above described plural types of particle groups in space defined between them are disposed two-dimensionally. However, it may also be arranged in such that the above described space contains a first section space enclosing the plural types of particle groups, and a second small section space part adjacent to the first section space and enclosing at least one particle group having a different color from that of the plural types of particle groups. Even in the case where multiple colors are displayed by combining adjacent section spaces with each other, a display color displayed in the respective section spaces can be increased. Accordingly, the number of cells forming one picture element can be reduced. Into the second small space part, the above described plural particle groups maybe enclosed as in the first section space.
Moreover, an electric field generator applies an electric field having intensity corresponding to the particle types to be transferred or moved, after the plural types of particle groups are moved to one of substrates. According to this arrangement, it becomes possible to achieve control for selectively moving or transferring particles, whereby color image quality can be improved. Particularly, in the case where white particles are enclosed as a particle having a reverse polarity, and an electric field is applied such that the white particles shift to the side of the display screen, wherein the white particles have adhered previously to the side of display screen. Accordingly, when a color image is formed, there is no unevenness on the display surface, so that colors in the image appear brilliantly.
Furthermore, it may be arranged such that a memory for storing a present display color determined by at least one particle group transferred or moved to one of substrates having translucence is provided; and the electric field generator determines a manner for applying an electric field for displaying the following display color on the basis of the present display color stored in memory as well as the following color for display, and applies the electric field in accordance with the manner determined for applying the electric field. In case of displaying animation, it is more natural for a viewer that color change for each cell unlike the above described mode. Accordingly, the present mode is particularly effective for movies.
An image display medium according to the present invention described is provided with a pair of substrates at least one of which is substantially transparent, space defined between them containing a gas or substantial vacuum; and a plurality of types of particle groups enclosed in the space defined between the substrates in such a manner that they can be transferred between the substrates by means of an applied electric field, types of particle groups, which having charge properties that are the same as one another. As mentioned above, the above described van der Waals force (intermolecular force) acts between the substrates and the particles. Thus, it becomes possible to provide an image display medium having memory function, being easily handled, and capable for displaying multiple colors.
An image display controller is provided with a mounting assembly for detachably mounting the above-mentioned image display medium to the electric field generator, and the electric field generator for applying an electric field having an intensity in response to types of particle groups to be moved or transferred in the image display medium mounted to the mounting assembly. Hence, it is possible to store an image stored after the image display medium has been mounted, and then dismount and transport the medium to another location, wherein the image is maintained in the medium, due to its memory function.
Furthermore, it may be arranged in such that the above described electric field generator, applies the electric field in such a manner that particles collide with the substrate at a predetermined collision speeds less than that at a level which will cause particle deformation as a result of collisions with the opposing substrate. In this case, it may also be arranged such that the electric field generator applies a strong electric field required for initially pulling particles away from a substrate during an initial period, wherein the particles were initially adhered to the substrate.